Sydney is about to record its driest autumn spell on record before an east coast low brings heavy falls near 100mm. In the past 29 days Sydney has only received 0.6mm of rain. By tomorrow morning the city will have recorded the driest 30 day autumn spell on record. The previous record holder was 1.2mm in March-April 1980. Sydney's records go back to 1859. The beautiful, mostly sunny weather has been due to a dominance of high pressure over New South Wales. All that will end on Wednesday as a low pressure trough approaches from the west. On Wednesday skies will be cloudy with areas of light rain developing across the Sydney Basin, likely providing 5-10mm. However this is only a foretaste of the rain to come. An east coast low will develop on Thursday and linger into Friday, providing periods of heavy rain, strong winds and cooler temperatures. By Friday night Sydney is likely to have been drenched by approximately 100mm, with the risk of flash flooding. This system could bring the May average rainfall of 121mm in one fell swoop. On Saturday wind and showers will ease significantly as the low moves away from the coast. Sunday will then be calmer, with sunny spells and the chance of showers, most likely on the coastal fringe. The drenching will provide a boost to many parched gardens while putting a halt to the use of the hills hoist until next week.